I'm getting "Invalid or Expired Token"?

The "token" is the permission that Twitter gives TweetCaster to allow your account to work in the TweetCaster app.

If you remove your account from TweetCaster (tap the four squares in the upper left, long-press your avatar and select Sign Out) then remove the current validation from Twitter ( go to https://twitter.com/settings/applications and revoke access for the TweetCaster application), then add the account back, it should request a new token and hopefully correct the login issue.